Slag discharged from a blast furnace of a steel plant is cooled and then crushed by a crusher and utilized as a mixed member of concrete or asphalt. As a method of cooling the slag, for example, a method of forcibly cooling the slag by spraying water thereto or the like is employed. When cooling the slag, a large amount of steam is intermittently generated and released into the atmosphere as it is without utilizing heat of the steam, resulting in energy loss.